Lidar Error Dictionary

By NGP Standards and Specifications

The purpose of this dictionary is to provide standardized language to describe commonly encountered errors associated with lidar datasets submitted to the USGS data validation unit for ingestion into the National Map. This document will improve communication between 3D Elevation Program operations personnel, the project’s point of contact, contributors, partners, and contractors.

Download a pdf copy of this data dictionary

The error dictionary is in two parts--Part 1 (this page) covers Breakline and Classified Point Cloud deliverables. Part 2 covers Digital Elevation Model, FGDC XML Metadata, Reports, Spatial Metadata, and Raster Metadata deliverables.

Part 1

Breakline Deliverable

Breakline Enforcement Errors

Geometry Errors

Missing Feature Errors

Spatial Reference Errors

Classified Lidar Point Cloud Deliverable

Collection Errors

Header Property Errors

Point Data Record Errors

Point/Header Mismatch Errors

Point Misclassification Errors

Part 2:

Digital Elevation Model Deliverable

FGDC XML Metadata Deliverable

Reports Deliverable

Spatial Metadata Deliverable

Raster Metadata Deliverable

* Click on an image for a larger view

Deliverable: Breaklines

Error type: Breakline Enforcement

Error subtype: n/a

Aerial photograph, digital elevation model, and cross section illustrating a waterbody with an unenforced breakline. The surface

Error definition: Appears as though the breakline was not enforced to the water feature, bridge, island, or structure correctly.

Error type: Geometry Error:

Error in feature’s breakline shape, length, or elevation.

Error subtype: Downstream Constraint

Digital elevation model illustrating an error in downstream constraint. The water surface ramps upward in the downstream directi

Definition: Water feature does not present a gradient downhill surface that follows the immediately surrounding terrain.

Error subtype: Overextended Breakline

Definition: Breakline that goes beyond the full extent of the water body.

Error subtype: Under-extended breakline

Under-extended breakline lidar error — Lidar error dictionary figure 4. In this digital elevation model (A), a breakline (blue) does not follow the edge of the waterbody but instead is underextended. The intensity image (B) clearly shows water beyond the breakline. This breakline is also underextended in places (see lidar error dictionary figure 3).

Definition: Breakline that underrepresents the full extent of the water body.

Error subtype: River/Stream segments

Discontinuous stream/river segment breakline lidar error — Lidar error dictionary figure 5. Digital elevation model (A) and intensity image (B) showing a discontinuous stream segment that should have been collected as a single water feature.

Definition: A stream or river feature that is broken up into discontinuous segments when it should be a single collected water feature.

Error subtype: Z-Values

Z-values too low lidar error — Lidar error dictionary figure 6. Elevation cross section (B) showing Z-values that are well below the terrain instead of being at or immediately below the surrounding land surface. Top image (A) is a digital elevation model showing the waterbody (blue), breakline (blue line) and cross section location (yellow double line).

Definition: The elevation value of a breakline’s vertices are not at or below the immediately surrounding terrain. This can result in waterbodies that appear to be sunken or floating.

Error subtype: Z-Enabled

Breakline is 2D, not 3D, lidar breakline error — Lidar error dictionary figure 7. Screen capture showing attributes of a breakline that is a 2D feature, not a 3D feature with Z-values as required.

Definition: A breakline that has no elevation values and is a 2D feature. A 3D feature is required.

Error type: Missing Feature:

A breakline is absent for a feature that requires breakline collection. This can be a water feature, structure, bridge, part of a breakline, or island.

Error subtype: Breakline

Missing breakline feature lidar error — Lidar error dictionary figure 8. Digital elevation model showing a missing feature. The blue breakline should have continued along the river.

Definition: A segment or piece of a breakline on a feature that is missing.

Error subtype: Bridge

Missing bridge breakline lidar error — Lidar error dictionary figure 9. Digital elevation model (DEM) (A), point cloud colored by elevation (B), and classified point cloud (C) showing a bridge that is classified correctly, as can be seen in the center and right images, but lacks sufficient breaklines leading to artifacts in the DEM.

Definition: There is a bridge artifact, not caused by a point misclassification, that requires a breakline in order to remove it completely.

Error subtype: Island

Missing islands breakline lidar error — Lidar error dictionary figure 10. Digital elevation model (A) and classified lidar point cloud (B) showing an island (in red ellipse) that is classified correctly but is missing a breakline leading to an uneven water surface. The water surface should be flat. This island meets contracted minimum collection size for breaklines.

Definition: There is an island greater than the contracted collection size that is not encompassed with a breakline.

Error subtype: River/Stream

Missing stream/river breakline lidar error — Lidar error dictionary figure 11. Digital elevation model showing a river (in red polygon) that is not encompassed with a breakline and so is left unflattened. This river meets contracted collection size for waterbodies requiring breaklines.

Definition: There is a river or stream that meets the contracted collection size that is not encompassed with a breakline.

Error subtype: Structure

Definition: A breakline is absent for a building or non-organic feature that requires breakline collection or a breakline is needed to remove artifacts.

Error subtype: Waterbody

Missing waterbody breakline lidar error — Lidar error dictionary figure 12. Digital elevation model showing a waterbody not encompassed with a breakline. This waterbody meets the contracted collection size for waterbodies requiring breaklines.

Definition: Spatial Reference information is incorrect or is not correctly reported for some or all Breaklines.

Error type: Spatial Reference

Error subtype: n/a

Definition: Spatial Reference information is incorrect or is not correctly reported for some or all Breaklines.

Deliverable: Classified Lidar Point Cloud

Error type: Collection:

Errors that are related to the processing and creation of lidar point cloud data.

Error subtype: Corrupt File

Definition: LAS files will not load into visualization software or there is an invalid data type.

Error subtype: Data Void

Point cloud data void lidar error — Lidar error dictionary figure 13. Aerial imagery (A) and classified lidar point cloud (B) showing a void in the point cloud (green area in B).

Definition: Lapses in lidar data collection in the point cloud that are not due to surface environmental conditions. Examples would include but are not limited to lapse in coverage due to atmospheric interference, GPS outages, or cloud cover.

Error subtype: Density

Low density lidar error — Lidar error dictionary figure 15. Classified point cloud (A) with an area of lower point density that detrimentally affects the digital elevation model (DEM) (B). An area with TIN artifacts can be seen in the DEM in the area of lower density.

Definition: Point density for specific file(s) appears to be excessively high or low compared to the expected values.

Error subtype: LAS Version

Definition: Lidar point cloud data is stored in a version of LAS format inconsistent with Lidar Base Specification, American Society for Photogrammetry and Remote Sensing (ASPRS) guidelines, or contract/task order.

Error subtype: Relative Accuracy

Intraswath relative accuracy lidar error — Lidar error dictionary figure 17. Lidar point cloud by channel (A) and elevation cross section (B, along double-line in left image) showing an elevation error between intraswath channels. The vertical difference is greater than one foot.

Interswath relative accuracy lidar error — Lidar error dictionary figure 16. Classified point cloud (A) and elevation cross-section (B, taken along yellow double line in top image) illustrating an elevation mismatch between swaths (red and green).

Definition: Issues pertaining to the geometric accuracy of the point cloud. These errors can be related to the interswath and intraswath precision and may or may not have visual indication in the DEM surface.

Error type: Header Properties:

The LAS file header characteristics are incorrect according to the LAS version, Lidar Base Specification, American Society for Photogrammetry and Remote Sensing (ASPRS) guidelines, or contract/task order.

Error subtype: EVLR/VLR

Definition: Incorrect treatment of the Extended Variable Length or Variable Length Records. Typically, when one or many records are not superseded.

Error subtype: File Creation

Definition: The file creation (day and/or year) was incorrectly populated.

Error subtype: File Source ID

Definition: The file source ID is set incorrectly.

Error subtype: Generating Software

Definition: The type of software used to generate the LAS files was incorrectly populated.

Error subtype: Global Encoder

Definition: The global encoder value is set incorrectly.

Error subtype: PDRF/PDRL

Definition: The Point Data Record Format or Point Data Record Length are set to the incorrect value.

Error subtype: Spatial Reference

Definition: The Coordinate Reference System information is incorrect for some or all LAS files.

Error subtype: System ID

Definition: The system ID is not populated with an acceptable value.

Error subtype: XYZ Scale Factor

Definition: Scale factor not appropriately populated or applied.

Error subtype: XYZ Offset

Definition: Overall offset for point records is an incorrect value.

Error type: Point Data Records:

Properties of the LAS Points are incorrect for some or all LAS files according to applicable USGS Lidar Base Specification, American Society for Photogrammetry and Remote Sensing (ASPRS), or contract/task order.

Error subtype: Classification Flags

Improper application of withheld bit flag lidar error — Figure 19. Geometrically valid points flagged as withheld (A). Colorized points in A are geometrically valid as seen in B.

Not properly classified overlap points lidar error — Lidar error dictionary figure 18. Points flagged as overlap are not properly classified. Plan view (A) and cross-section (B) in overlap area shows points remaining unclassified.

Definition: Classification flags are set incorrectly. This includes overlap, withheld, key-point, and synthetic flags. Text field will be used to specify.

Error subtype: Edge of Flight Line

Definition: The Edge of Flight Line data bit is set incorrectly.

Error subtype: GPS Time

Definition: The Global Positioning System of a LAS file is incorrect. Either incorrect GPS time format, incorrect date, or missing information.

Error subtype: Intensity

Inconsistent normalization of intensity values lidar error — Lidar error dictionary figure 20. Image of LAS intensity values showing inconsistent normalization.

Definition: Intensity values of the LAS points are not normalized to 16 bit and/or did not use linear normalization.

Error subtype: Point Source ID

Incorrectly set point-source IDs lidar error — Lidar error dictionary figure 21. A lidar point cloud colored by Point Source ID. The Point Source IDs in this dataset have been set incorrectly and do not align with the flight lines (purple).

Definition: Point Source ID values are set incorrectly.

Error subtype: Return 0

Definition: A pulse return of 0 has been found in some or all LAS files.

Error subtype: Scan Angle

Definition: The Scan Angle Rank appears to be an incorrect value.

Error subtype: Scan Direction Flag

Definition: The Scan Direction Flag bit is set incorrectly.

Error subtype: XYZ Position

Definition: There is an issue with the X, Y, Z values in some or all the LAS files.

Error type: Point/Header Mismatch:

There is a discrepancy between the information recorded in the points and their respective header information.

Error subtype: Point Count

Definition: Point count header information does not match the actual point count.

Error subtype: Return Counts

Definition: Return count header information does not match the actual return count.

Error subtype: Returns

Definition: Return header information does not match the actual returns.

Error subtype: XYZ Min/Max

Definition: The X, Y, and/or Z header information does not match the X, Y, and/or Z point information.

Error type: Point Misclassification

Error subtype: n/a

Definition: There is an incorrect classification of points. Text field will be used to specify.

Point misclassification lidar error — Lidar error dictionary figure 22. High noise from reflective surfaces on a runway (aerial photograph, A) have been left in the ground classification (digital elevation model, B).